This invention relates to a two-circuit master cylinder unit for brakes, wherein a first piston is actuated in the usual manner by applying thereto an external force and a second piston is exposed to the pressure generated by the first piston. The unit comprises a switching device which, in case one of the circuits fails, changes its state and acctuates a warning device.
A unit of the afore-outlined type is known and is disclosed, for example, in the leaflet Bremesen-Service, published by ATE (Alfred Teves GmbH, a German company of Frankfurt/Main), 1969 Issue, page 01.6, FIGS. 19 and 20. In the structure disclosed in this leaflet there is provided an additional auxiliary piston, the two opposite work faces of which are exposed to the pressure in the one and the other circuit. If one circuit fails, the auxiliary piston is displaced, whereby a contact device is actuated.